The reflecting telescope is limited in contrast performance due to the spurious light levels induced by the central obstruction caused by the secondary mirror. This obstruction causes the impulse response or diffraction pattern to have higher side lobe levels than an unobstructed aperture. The additional light energy distributed in the region outside the main lobe response causes reduced contrast on extended objects such as planets and nebula. If the central obstruction were not present, the optical performance would be nearly perfect for an on-axis point like object (achromatic and ideal diffraction pattern) with contrast limited only by light scattered at the primary mirror surface and by deviations from an ideal parabolic surface. This is why a good refractor telescope is considered superior to the Newtonian or other reflecting telescope designs, by some, for planetary and other applications requiring low spurious noise levels.
Examples of prior art reflecting telescopes including auxiliary optical systems are U.S. Pat. Nos. 2,628,529 to Braymer, issued Feb. 17, 1953; 3,598,468 to Perry, issued Aug. 10, 1971; 3,667,827 to Lawrence, issued June 6, 1972; and 3,752,559 to Fletcher, et al, issued Aug. 14, 1973.
The Braymer patent discloses a reflecting telescope including an auxiliary optical system in which the primary image is formed by a right cone of rays whose base is the effective area of the primary mirror and whose axis is coincident with the principal axis of the instrument. The Perry patent discloses an optical system for a microscope including a spherical mirror tilted a few degrees relative to the optical axis and a plane plate having a transparent refractive portion in the path to the mirror and a reflective surface on the path from the mirror, with the plate tilted so that its refractive portion corrects astigmatism which results from tilting of the mirror. The Lawrence patent discloses a tele-objective in which the diffraction effects produced by the central obstruction effecting the image quality are very small. A relatively small positive achromatic doublet provides correction of aberrations of a concave spherical primary mirror. The Fletcher, et al, patent discloses a Ritchey-Chretien telescope responsive to images located off the telescope optical axis and includes a transparent plate positioned in the ray path of the image. The flat plate has a tilt angle relative to the ray path that compensates substantially for astigmatism introduced by the Ritchey-Chretien telescope.
None of the prior art patents provide a means for correcting for the central obstruction presented by the secondary mirror of a reflecting telescope.